The disclosure of Japanese Patent Application No. 2003-371354 filed on Oct. 30, 2003, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a failsafe hydraulic circuit designed to avoid simultaneous engagement of two friction engagement elements (e.g., clutches, brakes, etc.) incorporated in, for example, a speed change apparatus of a motor vehicle or the like.
2. Description of the Related Art
A speed change apparatus installed in a motor vehicle or the like has a plurality of clutches and brakes that are engaged and disengaged on the basis of the engaging pressures supplied to hydraulic servos. Among these clutches and brakes, the clutches and brakes that need to avoid simultaneous engagement are provided with failsafe hydraulic circuits.
A known failsafe hydraulic circuit as mentioned above is equipped with a failsafe valve and an oil pressure switch (oil pressure detector device), as disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 2003-49937.
For example, an oil supplying passage to the hydraulic servo of a specific friction engagement element is provided with a failsafe valve that operates to prevent the supply of oil pressure to the specific hydraulic element, using as a signal pressure the oil pressure supplied to the hydraulic servo of a friction engagement element that should not engage simultaneously with the specific friction engagement element in order to achieve a related shift speed. In order to check the operation of the failsafe valve, line pressure is connected to a portion of the valve, and the introduction and shutoff of the line pressure is detected by the oil pressure switch.
However, the above-described failsafe hydraulic circuit has the following problem. That is, although the oil pressure switch is able to detect whether the operation of the failsafe valve is normal, it is impossible to acquire further information, for example, information regarding the engaging pressure supplied to the hydraulic servo or the like.